1. Field of the Invention
The present invention relates to a copper powder wherefrom an electrically conductive paste showing low viscosity even at a high filling rate is prepared.
2. Description of the Related Art
When a thick film circuit substrate is prepared by a screen printing of an electrically conductive paste on an insulation substrate, a paste of silver type has been mainly used as the electrically conductive paste although there is a tendency of using a paste of copper type as well. This is because the paste of copper type has the following advantages as compared with that of silver type.                (1) Since migration hardly takes place, short circuit rarely happens.        (2) Since resistance to soldering is excellent, reliability is high.        (3) Cost reduction is possible.        
The copper paste having such advantages can be prepared by dispersion of copper powder having a particle size of around 0.1-1 μm in a vehicle (resin).
With regard to a method for the manufacture of copper powder, there have been known mechanical grinding, atomizing method where melted copper is sprayed, electrolytic deposition method to cathode, vaporization-deposition method, wet reduction method and the like. Although each of them has its own merit and disadvantages, a wet reduction method is mainstream for the manufacture of copper powder for electrically conductive paste because it is able to relatively easily give fine powder having a particle size suitable for the paste. A method for the manufacture of copper powder by a wet reduction method is mentioned, for example, in JPA-Hei-4-116109, JPA-Hei-2-197012 and JPA-Shou-62-99406.
Gist of a method for the manufacture of copper powder by a wet reduction method is that copper hydroxide precipitated in water is subjected to a primary reduction to cuprous oxide and then the cuprous oxide is subjected to a secondary reduction to give metal copper where glucose or the like is used as a primary reducing agent while hydrazine hydrate or the like is used as a secondary reducing agent. At that time, particle size and particle shape of the resulting copper powder can be controlled by adjusting the conditions for a precipitating step of copper hydroxide and primary and secondary reducing steps and there is an advantage that a product having a particle size suitable for electrically conductive paste can be manufactured in a stable manner. In JPA-2000-144217, the present inventors already proposed a method of carrying out an oxidizing treatment by blowing-in of oxygen-containing gas between the primary and the secondary reducing steps. As a result of the oxidizing treatment, copper powder having a uniform particle size can be prepared and control of particle size and of particle shape can be made much more precise.
Even if the copper powder having a particle size suitable for electrically conductive paste is manufactured, there is still a problem in the wet reduction method when the resulting copper powder is to be used for preparing an electrically conductive paste having an appropriate viscosity. Viscosity of the used resin itself, filling rate (filler value) of copper powder and particle size distribution participate in the viscosity of an electrically conductive paste, however, in the copper powder by a wet reduction method, the viscosity of the paste tends to be increased. That is, it has been found that only an appropriate control of the particle size (even if it can be done) has a limit for lowering the viscosity of the electrically conductive paste in the copper powder by a wet reduction method.